The sintering mechanism of fully dense and highly coercive Nd-Fe-B magnets from the recycled HDDR powders reprocessed by spark plasma sintering

The goal of this study was to justify the reprocessing of recycled HDDR Nd-Fe-B powders with spark plasma sintering (SPS) and to investigate the dependence of the final magnetic properties on SPS and thermal treatment. The initial recycled HDDR powder of the composition Nd(13.4)Dy(0.6)Fe(78.6)B(6.1)Nb(0.4)Al(0.7 )with 4760 ppm O-2 content, coercivity (H-Ci) = 830 kA/m, and large particle size distribution <50-600 mu m, was treated using the SPS parameters as follows: T= 650-850 degrees C for 1-5 min and 50 MPa pressure. The post SPS thermal treatment was performed at 750 degrees C for 15 min in vacuum. The optimal SPS conditions at 750 degrees C for 1 min, yielded fully dense magnets with the coercivity H-Ci = 1060 kA/m, which was boosted to 1160 kA/m after the post-SPS thermal treatment. The grain growth was obvious in samples SPS-ed above 800 degrees C and subsequently, the H-Ci was reduced. The SPS reprocessing beyond 850 degrees C was found to be detrimental to the overall magnetic properties due to the formation of bimodal grain size distribution origination from the abnormal grain growth (700-2600 nm). The redistribution of Nd-rich grain boundary phase between the Nd2Fe14B matrix grains in thermally treated magnets and the relaxation of the internal stresses induced via SPS are the possible reasons for the H-Ci surpassing the H-Ci of the starting recycled HDDR powder. It was shown that the SPS consolidation technique is suitable for producing fully dense nanograined bulk Nd-Fe-B magnets, with coercivities even exceeding the initial HDDR-powder and is one of the most suitable routes for revitalizing the Nd-Fe-B scrap magnets. (C) 2018 The Authors. Published by Elsevier B.V.